[Chore] Invalid CSS syntax generated by variables with no default value

[ko2in]

Description

Some variable files contain the variables with no default value, which resulted the LESS file to generate the invalid CSS syntax. Example: a variable @searchMinWidth with empty value would generate an invalid CSS declaration min-width: ''.

Fomantic-UI/src/themes/default/modules/dropdown.variables

Line 222 in df26dde

@searchMinWidth: '';

Which produces an invalid CSS like:

Literally, the min-width declaration is not required at all, if there's no value for that. The browsers will be ignored any invalid CSS declaration, however it's still an error and violating CSS rules, and thus should be avoided.

There can be several variable files which could produce this outcome and all of these variables should be reviewed and remove the corresponding CSS declarations which bound to those variables.

I proposed this for the following reasons:

• To avoid any undesired outcomes when the browsers changed their behavior for dealing with invalid CSS syntax in the future
• The user could avoid having the alert for any invalid CSS syntax when using the CSS linter
• To keep FUI following CSS specifications as much as possible and have valid syntax

[lubber-de]

Those invalid values might only exists in the default theme. Custom themes could potentially have set valid values for those variables, so we cannot remove them in general without being a breaking change.
We should rather look into some feature/linter on LESS build time to not render CSS attributes if they are invalid.